Megger TORKEL 910, User Manual

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TORKEL 900-SERiES ZP-CS01E CS033663DE
8 HOW TO OBTAiN THE DESiRED CURRENT
8
H
ow to obtain the desired
current
8.1 When a single TORKEL
isn’t enough
TORKEL regulates current by lowering its internal re-
sistance as the voltage drops. However, the resistance
elements built into TORKEL impose a limit beneath
which further lowering is impossible.
▪
You must make some simple calculations before starting
a test to find out whether or not TORKEL will be able to
provide the desired load current.
▪
You must also make certain that TORKEL will be able to
sustain this current until the test ends.
When a single TORKEL cannot provide the current you
need, you can:
▪
Connect one or more TXL Extra Loads to TORKEL.
▪
Connect two or more TORKELs in parallel.
▪
Connect two or more TORKELs and two or more TXL
Extra Loads into a single system.
When two or more TORKELs are connected into a sin-
gle system, you will normally use the “External current
measurement” function.
TXL connected to TORKEL
TXLs are resistive loads which are unable to provide
any sort of regulation. Regulation is provided by
TORKEL which measures the total current and keeps
it constant. See the chapter headed “External cur-
rent measurement” which shows how to connect the
TXL(s) and TORKEL(s).
When TXL Extra Loads are connected to TORKEL, you
must check:
▪
That the current flowing through the TXLs when the test
is started is not higher than intended.
▪
That TORKEL has enough regulation capability a) to
compensate for the drop in current through the TXLs at
the end of the test and b) to set the current to the correct
value at the beginning of the test.
How many units are needed?
Automatic calculation - TorkelCalc
in TORKEL Viewer (not included with TORKEL 910)
you can use TorkelCalc to figure out how many
TORKEL and TXL are needed.
See"4.2 TORKEL Viewer" on page 19.
Manual calculation
1. Number of TXLs – Current flowing
through TXL(s) at beginning of a test
At the beginning of the test, as high a percentage as
possible of the current must flow through the TXLs,
thereby providing the TORKEL(s) with as much reserve
regulation capability as possible. However, the current
through the TXLs must not, of course, exceed, the
desired current value (A).
Tip
Remember that the internal resistances of the
TXLs can be set manually. For accurate calcula-
tion, add the cable resistance to the internal
resistance.
▪
The current in an individual TXL can be obtained by dividing
the voltage at the beginning of the test by the internal
resistance of the TXL in question (see tables below).
▪
Calculate the number of TXLs that you can connect
without exceeding the desired total current.
2. Current flowing through TXL(s) at final
voltage
▪
Multiply the total current through the TXL(s) which you
obtained in step 1 above by the final voltage, and then
divide by the voltage at the beginning of the test.
3. Number of TORKELs – for the current
regulation
The TORKEL or TORKELs in the system must regulate
the current to the desired value and compensate for
the drop in current through the TXL(s) that occurs at
the final voltage.